EP-A-0 744 586 has disclosed a heat-transfer element, for example a plate or tube, with a large heat-transferring surface in the form of copper foam, for use in a heat exchanger, in order to improve the heat transfer. An element of this type is produced by using a vapour deposition process to deposit a powder of copper oxide on a plastic foam which has previously been provided with a suitable adhesive. The foam which has been prepared in this way is then arranged under slight pressure on a plate or tube, which has likewise previously been covered with a copper oxide powder, in order in this way to form a composite element by sintering. After pyrolysis of the plastic foam, the copper oxide is reduced to form copper.
A heat exchanger of the type described above is used, for example, in what are known as thermo-acoustic heat engines. In a heat exchanger of this type, a first heat circuit is formed by a flow of a first fluid, such as a gas or liquid, through generally a plurality of flow passages. A second heat circuit comprises a flow of a second fluid, generally a gas (air, argon), through the porous flow body, which flow body surrounds the flow passages over a certain area. The direction of flow of the second fluid through the flow body is generally virtually perpendicular to the direction of flow of the first fluid in the flow passages. The porous flow body is in heat-exchanging contact with the outer wall of the flow passages. Heat is transferred, for example, from the first fluid to the inner wall of the flow passages and is carried to the outer wall as a result of conduction in the wall material. At the outer wall, heat transfer to the porous flow body takes place through radiation and conduction. Heat conduction takes place in the porous flow body. When there is only a flow body made from metal foam, this heat conduction is limited, and consequently solid lamellae made from a material with good conductivity are sometimes provided in the metal foam in order to increase the heat conduction. Transfer of heat from the flow body to the second fluid likewise takes place by means of radiation and conduction. The efficiency of the heat transfer overall is dependent, inter alia, on all these transitions, the transfer from the flow body to the second fluid or vice versa—generally the heat transfer on the gas side—in particular possibly representing an inhibiting factor.
It has now been found that, although the use of a metal foam, optionally in combination with lamellae or fins, offers an enlarged heat-exchanging surface area and possibly increased conduction, the flow resistance is relatively high, so that the overall performance, expressed as the ratio between heat transfer and flow resistance, is inferior to that of a conventional heat exchanger with only fins or lamellae. In many cases, an increase in the heat transfer when using a metal foam goes hand-in-hand with a disproportionate increase in the flow resistance.
U.S. Pat. No. 4,245,469 has disclosed a heat exchanger in which a porous metal matrix is arranged in a flow passage through which a heat-transferring medium flows. It is stated that this metal matrix has a greater density in an area which is perpendicular to the direction of flow, so that the internal heat transfer coefficient is increased in this area, where the temperature of the environment is much higher than at the end of the passage. To minimize the reduction in volume of the heat-transfer medium which would be produced with a passage of constant diameter, the diameter is increased at the location of the said area. A design of this type aims to improve the internal heat transfer.
Furthermore, DE A1 39 06 446 has disclosed a heat exchanger in which a foam, for example of aluminium, is arranged in a flow passage. If desired, the pore size in this foam may be varied, i.e. the number of pores may vary.